Full head of steam

With energy prices the highest in recent history and environmental concerns unlikely to slip off the political or business agenda anytime soon, the pressure is still on to maximise plant efficiencies and minimise costs. That applies to all plant but steam is right up there, being responsible for an estimated 40% of consumption, averaged across all industries. Clearly, boiler houses need our attention, and not just in terms of maintenance, essential though it is, but also periodic reappraisal – treating them like any other machine system. To do so, however, engineers and technicians need more than a passing understanding of how to assess overall boiler plant and associated steam distribution and condensate return system efficiencies. They also need to know what good looks like – and the lowest-cost, highest-return technologies and methods likely to get them there. And although it's important to install waste heat recovery equipment and new process automation systems at one level, and inspect steam traps, valves and pipework connections, and insulation at the other, there are some other steps. For Nigel Egginton, managing director of specialist EBE Engineering, those absolutely include first looking at the viability of existing boiler plant. "The departure point for considering improvements to steam-raising boilers – such as adding economisers or improvements to the burners – has to be the physical condition of the boiler vessels themselves," he rightly observes. Classic boiler inspection techniques include: ultrasonic thickness surveys and radiography (conventional or digital). Egginton describes the former as using "software and experience" to gather information about vessel wall thicknesses. That sounds like a black art, but, in the right hands, today's ultrasonic NDT (non destructive testing) systems are very good at revealing wear patterns and localised corrosion losses. Radiography is then better at finding corrosion fatigue cracking and weld quality issues – not just around boiler walls, but also riser tubes and associated piping – all without removing the boiler lagging. Nevertheless, a good starting point remains nothing more sophisticated than visual inspection – examining boilers and associated equipment, looking for evidence of mechanical damage and wear. As Egginton says, this is particularly important for industrial boilers that regularly experience harsh service conditions. And the same is true of the steam distribution system – checking for leaks but also malfunctioning steam traps, etc. Experience suggests that, depending on maintenance routines and equipment types, 15-25% of steam traps will have failed open or partly open in any factory, wasting thousands of pounds worth of energy and water, plus the associated maintenance expense. If your boiler plant passes muster, then it's time to consider the burners and their ability to efficiently burn fuel, suggests Egginton. "Burners are considered efficient if they leave low levels of unburned fuel while operating at low excess air levels," he asserts. And he makes the point that this is not the same as assessing boiler efficiency – which, while related, is a matter of comparing energy output to energy input. Incidentally, assessing fuel stack excess air alone is not enough. As Sharon Kuligowski, managing director of Dunphy Combustion, reminds us, excess air measurements tell you nothing about burner efficiencies, the economiser (if fitted), the boiler itself or the steam distribution systems. "Everyone knows that the fuel-air mix is key – that's why we measure it. But they don't seem to realise you need stoichiometric combustion. Yes, you want some excess oxygen to prevent carbon monoxide formation, but as low as possible, so you're not wasting energy raising the flue gas temperature and increasing NOx emissions." Egginton agrees, and points to the importance of modern technology in getting these under control. "Burners have improved greatly in recent years," he comments. "Older burners had a series of cams and levers calibrated to provide specific amounts of air for particular firing rates." Being mechanical items, these wear and need regular servicing and recalibration. "But newer burners generally use frequency-controlled electric motors and cams to open air vents and run fans at optimum speeds, improving performance by up to 3%." In fact, he contends that even burners older than, say, five years are typically 1-2% less efficient than their current counterparts. Why? "Because the latest burners re-circulate flue gases to ensure optimum combustion. They are also fitted with sophisticated electronic control systems that monitor the components of the flue gas, and make adjustments to fuel and air flows to maintain conditions within tightly-specified parameters." Not only that, but they also offer greatly improved turndown ratios, enabling good efficiency and emissions figures over greater operational ranges. As Kuligowski puts it: "Today, it's about managing air flow up front – starting by monitoring the fuel-air mix and distribution in the burner and mapping requirements to the boiler load directly." The only caveat: any air leaks in the combustion chamber will wreak havoc with any combustion control. That said, boilers without any kind of economiser (used to recover otherwise wasted flue gas energy, via heat exchanger coils, to preheat the boiler feed water) are typically only 88% efficient in burning fuel. Most industrial plants favour non-condensing economisers, designed to maintain the flue gas temperature above its dew point and so prevent corrosion of the ducting. However, condensing economisers – which typically heat a larger volume of water to a lower temperature – generally offer improved efficiency, because they also extract latent heat of vaporisation (around 9% of the initial fuel energy content) and reduce the outlet exhaust temperature. As for cost, Egginton suggests that a non-condensing economiser for a 5.5-tonne boiler producing steam at 7.5 bar would come in at £24,000-£25,000, giving a payback of less than two years. Fully condensing economisers require additional construction, but, as virtually all of the gas energy is recovered, paybacks are similar – typically two years – and then you're into a net gain. Boiler efficiency checklist - Evaluate your boiler capacity, average steam generation and demand, combustion efficiency, flue gas temperature, annual hours of operation and annual fuel consumption - Determine where heated water may be used – such as boiler makeup water heating, reheating, or domestic hot water or process water heating - Estimate the thermal requirements that can be serviced through installation of a condensing or non-condensing economiser. Calculate annual fuel energy and cost savings - Obtain an installed cost quotation and determine the cost-effectiveness of a non-condensing economiser or a condensing economiser - Review boiler condensate return system and ensure correct installation and operation of all piping, insulation and steam traps. Flash steam recovery packs £40,000 per year punch A Spirax Sarco flash steam recovery system is saving Jardin Corrugated Cases nearly £40,000 per year and has reduced the plant's CO2 emissions by 282 tonnes per year. What's more, the Ely, Cambridgeshire-based plant is reporting that the whole project was financed by an interest-free loan through the Carbon Trust. A spokesperson for Jardin explains that flash steam had been escaping from its 11,000 sq m manufacturing site for some time, wasting energy and creating a poor environmental image. Spirax Sarco designed, installed and commissioned a flash recovery system, and applied for the Carbon Trust loan on behalf of Jardin. The system saves 2.8 tonnes of CO2 per £1,000 spent, beating the Carbon Trust's loan criterion of a minimum two tonnes per £1,000 spent. At the heart of the new installation is a Spirax Sarco FREME (flash recovery energy management equipment) system that captures all usable heat in the condensate, returning it to the boiler. FREME works by passing condensate through a heat exchanger, where it heats the feed water on the high-pressure side of the boiler feed pumps, so preventing boiling and cavitation. Spirax Sarco also provided automatic boiler blowdown controls with heat recovery, boiler feedtank insulation and a packaged pump system to ensure that all condensate from the main corrugator is returned for recovery. As well as the savings in energy and carbon dioxide emissions, Jardin reports that the system is saving water and water treatment chemical costs. It also means no more manual blowdowns every day.